RxJS logic which solves a filter/merge issue

Question

This is more a logical problem then a RxJS problem, I guess, but I do not get it how to solve it.

[input 1] From a cities stream, I will receive 1 or 2 objects (cities1 or cities2 are test fixtures).
1 object if their is only one language available, 2 objects for a city with both languages.

[input 2] I do also have a selectedLanguage ("fr" or "nl")

[algo] If the language of the object corresponds the selectedLanguage, I will pluck the city. This works for my RxJS when I receive 2 objects (cities2) But since I also can receive 1 object, the filter is not the right thing to do

[question] Should I check the cities stream FIRST if only one object exists and add another object. Or what are better RxJS/logical options?

const cities1 = [
    {city: "LEUVEN", language: "nl"}
];

const cities2 = [
    {city: "BRUSSEL", language: "nl"},
    {city: "BRUXELLES", language: "fr"}
    ];

const selectedLang = "fr"
const source$ = from(cities1);
const result = source$.pipe(
    mergeMap((city) => {
        return of(selectedLang).pipe(
            map(lang => {
                return {
                    lang: city.language,
                    city: city.city,
                    selectedLang: lang
                }
            }),
            filter(a => a.lang === selectedLang),
            pluck('city')
        )
    }
    )
);
result.subscribe(console.log)

Answer 1

Thanks for your repsonse. It's great value for me. Indeed I will forget about the selectedLang$ and pass it like a regular string. Problem 1 solved

I'll explain a bit more in detail my question. My observable$ cities$ in fact is a GET and will always return 1 or 2 two rows.

leuven:
[ { city: 'LEUVEN', language: 'nl', selectedLanguage: 'fr' } ]

brussel:
[
  { city: 'BRUSSEL', language: 'nl', selectedLanguage: 'fr' },
  { city: 'BRUXELLES', language: 'fr', selectedLanguage: 'fr' }
]

In case it returns two rows I will be able to filter out the right value

filter(city => city.language === selectedLang) => BRUXELLES when selectedLangue is "fr"

But in case I only receive one row, I should always return this city.

What is the best solution to this without using if statements? I've been trying to work with object destruct and scaning the array but the result is always one record.

// HTTP get
const leuven: City[] = [ {city: "LEUVEN", language: "nl"} ];

// same HTTP get
const brussel: City[] = [ {city: "BRUSSEL", language: "nl"},
                          {city: "BRUXELLES", language: "fr"}
                        ];

mapp(of(brussel), "fr").subscribe(console.log);

function mapp(cities$: Observable<City[]>, selectedLanguage: string): Observable<any> {
    return cities$.pipe(
        map(cities => {
            return cities.map(city => { return {...city, "selectedLanguage": selectedLanguage }}
            )
        }),
        // scan((acc, value) => [...acc, { ...value, selectedLanguage} ])
    )
}